Some foreigners asked us what is the difference beteween arigatou gozaimasu versus arigatou gozaimashita?
Arigatou gozaimashita （ありがとうございました）” is used if something has already been done for you. It refers to something that has been done in the past.
Arigatou gozaimasu （ありがとうございます） is used to express your gratitude. You can use it to express something that will be done , such as thank you in advance in english.
|Situations||Arigatou Gozaimasu||Arigatou Gozaimashita|
|Now you are happy because of some nice treatments from other people and you want to say thank you very much||今日は本当にありがとうございます
kyou wa hontouni arigatou gozaimasu
kyou wa hontouni arigatou gozaimashita
|You want to say thank you to a person who always treats you nicely||いつもありがとうございます。
itsumo arigatou gozaimasu
|You want to say thank you to a person who gave you a nice present last month||X||この前はありがとうございました
kono mae wa arigatou gozaimashita
|You want to say thank you to a person who gave you a nice present yesterday||昨日はありがとうございます（可）
kinou wa arigatou gozaimasu (some people say these sentences)
kinou wa arigatou gozaimashita
Hope this helps!
Enjoy your stay in Japan!